A transfer case having a low gear range and a high gear range is commonly provided in a four wheel drive vehicle. The torque input from the transmission is passed to a transfer box input shaft. A differential, such as a Torsen® differential, can be provided in the transfer case to split torque between the front and rear wheels of the vehicle. The high range is used for normal road driving and also for off-road driving across dry, level terrain. The low range may be engaged for low speed maneuvering, such as reversing a trailer, negotiating steep slippery surfaces or boulder strewn terrain. The low range can also be used for extreme off-road conditions where progress in high range cannot be maintained.
In high range, a synchroniser sleeve in the transfer case couples the transfer box input shaft directly to the differential housing. The differential splits the torque between two side gears. One side gear is connected by splines and passes the torque to a rear output flange to drive the rear wheels. The second side gear is connected to a chain drive sprocket and passes the torque, via the chain, to the front output flange to drive the front wheels.
In low range, the synchroniser sleeve is moved and connects a planetary carrier to the differential housing. The torque from the transmission is now directed through a sun gear of an epicyclic gearset and, via the pinion gears and pinion gear shafts, into the planetary carrier. An annulus gear of the epicyclic gearset is secured inside the casing and generates the low range ratio, for example 2.69:1. The torque is then passed, via the synchroniser sleeve, to the differential housing where it is split between the two side gears.
The synchroniser sleeve is actuated by an electric motor driving a transfer fork along a linear path. To facilitate changes from the high range to the low range while the vehicle is on the move, an inner and outer blocker ring and a cone are provided. When the fork moves the sleeve in a first direction, the inner and outer blocker rings and the cone are pushed together and mesh at an angle, thereby operating as a synchromesh to provide smooth engagement.
The transfer case input shaft also has an outer blocker ring and a cone which are pushed together when the fork moves the sleeve in a second direction. The outer blocker ring and the cone also mesh together at an angle and operate as a synchromesh to provide smooth transition into the high range.
Providing a synchromesh increases the complexity and cost of the transfer case. Moreover, wear on the components in the transfer case can result in additional maintenance being required. The present invention seeks to address one or more of these problems. Embodiments of the invention may provide a method or a system which removes the need for a mechanical synchromesh to smooth the transition from a low range to a high range; and/or from a high range to a low range.